Polymer films are widely used for optical purposes. In particular, cellulose acylate films are commonly used as optical films for producing liquid crystal display devices of thin types at low cost since the cellulose acylate films can be used for protective films for polarizing filters and the like.
The above polymer films are mainly produced by solution casting methods. In the solution casting method, a polymer solution (a dope) containing a polymer such as cellulose acylate and a solvent is cast onto a moving support to form a casting film. The casting film is peeled off from the support as a wet film. The wet film is dried to form the polymer film.
The wet film is dried while being supported by plural rollers during the transportation. It is also possible to use a tenter device for drying the wet film. In the tenter device, the wet film is dried while both side edges of the wet film are held and stretched by holding members. However, in the above cases, after the casting film is peeled off as the wet film from the support, the drying levels may vary with portions of the wet film such as both side edges, a center portion, and front and back surfaces, resulting in curls in the both side edges of the wet film. The size of the curls increases as the drying proceeds.
However, such large curls may cause the holding defects such as foldings and ruptures of the wet film especially in using the tenter device where the side edges of the wet film are held by the holding members (for instance, clips). If the holding defects occur, the holding members cannot stably hold the side edges of the wet film. As a result, problems may arise such as uneven tension applied to the wet film, causing the reduction of the stability in the transportation of the wet film.
To prevent the holding defects in the tenter device, a film production apparatus which cuts off the side edges of the wet film between the peeling process of the wet film from the support and the winding process, and a film production apparatus provided with a curl preventing device are suggested. The curl preventing device has a pair of upper and lower guide plates disposed on both side edges of the tenter device close to the entrance thereof to guide the side edges of the wet film for preventing curling (for instance, see Japanese Patent Laid-Open Publication No. 11-090942). Moreover, a film production apparatus in which lubricating property and durability of the guide plates are enhanced by improving the surface material thereof is suggested (for instance, Japanese Patent Laid-Open Publication No. 2003-033933).
However, since the above film production apparatuses use the guide plates, abrasion of the guide plates is caused by the contact of the guide plates to the wet film. A part of the wet film may be shaved by the guide plates, and the shavings may be mixed into the wet film during the drying process. Accordingly, degradation of the optical property such as light scattering, and that of the planarity may occur.
An object of the present invention is to provide a method and an apparatus for producing a polymer film which prevent the holding defects by approximately flattening the curls in both side edges of the wet film without contacting the wet film so that the holding members can hold the wet film easily and stably to achieve excellent film transportation stability.